1. Field of the Invention
The present invention relates to a liquid crystal display device having an opening formed in an electrode, and more particularly to a liquid crystal display device which is capable of making display with an widened viewing angle in a making vertically aligned ECB (Electrically Controlled Birefringence) mode.
2. Description of a Related Art
A conventional liquid crystal display device will be described with reference to the accompanying drawings. The TN (Twisted Nematic) mode has been widely used in general as a liquid crystal display device. This TN-type liquid crystal display device has alignment layers, with mutually different orientations, provided at each of opposed surfaces of a pair of substrates holding a liquid crystal layer therebetween. The direction of the liquid crystal director remains in a twisted state between both substrates when no voltage is applied to the liquid crystal cell.
In the manufacturing process of the TN-type liquid crystal display device, a processing step has been required for rubbing the surface of the alignment layer in the predetermined direction with a silk cloth or the like, for aligning the liquid crystal director at the surface of the alignment layer composed of macromolecular film such as polyimide to orient the liquid crystal director (this processing step is referred to as a rubbing process).
However, since static electricity is generated during the rubbing process, the charges must be removed by post-processing to prevent undesirable effects, necessarily increasing the number of processing steps.
Further, in the TN-type liquid crystal display device, the viewing angle characteristics vary depending on the viewing direction. In some cases, a problem arises, the so-called image reversal problem, in which the displayed image looks inverted, so that the desired viewing angle characteristics are not maintained for all the viewing directions. This is due to the limitation inherent in TN mode that the desired viewing angle characteristics are related to the rubbing direction.
On the other hand, there is also known a liquid crystal display device called ECB in which the liquid crystal director on the surface of the alignment layer is oriented in the vertical direction with respect to the surface.
FIG. 1 shows a composition of a conventional liquid crystal display device using a vertical alignment layer. This device comprises: a TFT (Thin Film Transistor) substrate 2; a first vertical alignment layer 3; a liquid crystal layer 4; a second vertical alignment layer 5; an opposed electrode 6; and a second polarizing plate 8 all provided on a first polarizing plate 1. An opposed substrate 7 is provided between the opposed electrode 6 and the second polarizing plate 8. In this device, since the liquid crystal molecules at the surface of the alignment layer are oriented in a vertically aligned state due to the excluded volume effect, the liquid crystal director are accordingly oriented in a vertically aligned state, so that no rubbing process is required at this time. The polarizing axes of the first polarizing plate 1 and the second polarizing plate 8 mutually form an angle of 90 degrees (this state will be referred to as crossnicole hereafter).
In operation, light incident on the first polarizing plate 1 side then becomes linearly polarized light, and passes through the TFT substrate 2 and enters the liquid crystal layer 4.
When no voltage is applied between a pixel electrode (not shown) provided on the TFT substrate 2 and the opposed electrode 6, the liquid crystal molecules 4A of the liquid crystal layer 4 are arranged in the vertical direction, and the light having passed through the liquid crystal layer 4 remains linearly polarized light which is totally removed by the second polarizing plate 8. As a result, the incident light through the first polarizing plate 1 side does not pass therethrough.
Meanwhile, when a voltage is applied between the pixal electrode and the opposed electrode provided on the TFT substrate 2 (not shown) and the opposed electrode 6. Since the liquid crystal molecules 4A bend in accordance with the electric field, the light passing through the liquid crystal layer 4 exhibits birefringence along the liquid crystal molecular axis. So the light out of the liquid crystal layer 4 becomes elliptically polarized light, and is not removed by the second polarizing plate 8 and passes through the second polarizing plate 8.
However, according to such a conventional liquid crystal display device having the aforementioned electrode arrangement, problems arise as described below.
FIG. 2 is a plan view showing a driving state of the conventional liquid crystal display device, while FIG. 3 is a cross-sectional view along line D'--D' in FIG. 2. FIG. 4 is a drawing for describing the problems in the conventional liquid crystal display device, which is seen from the display screen when voltage is applied to the pixel electrode to pass the light.
In the driving state of the liquid crystal display device, there is a potential difference between the pixel electrode 2a and the opposed electrode 6, and an electric field arises between the opposed electrode 6, pixel electrode 2a and the gate bus lines 2b and 2c, respectively. The liquid crystal molecules 4A in the liquid crystal layer 4 located between the opposed electrode 6 and the pixel electrode 2a become bent in accordance with the strength of the electric field as shown in FIG. 2.
Accordingly, as shown in FIG. 4, the light illuminated from the back surface of the device passes through in the region of the pixel electrode 2a. At the other regions, meanwhile, a light-shielding film is formed to improve the contrast (this region will be referred to as a light-shielding region 8 hereafter), where no light can pass through.
In this case, for example, the liquid crystal director is not identical even in the region of one pixel electrode 2a where the light passes therethrough, so that a disclination line indicating the border of the difference of the liquid crystal director appears on each pixel. At this region, the transmittance is lower in comparison with other regions.
It is impossible to align the liquid crystal director uniformly when the liquid crystal is sealed between the vertical alignment layers, resulting in a nonuniform state. As a result, the liquid crystal director of each pixel at the initial conditions would be dispersed.
Therefore, even if a voltage of the same conditions is applied to each pixel, the disclination lines 9 will not arise at the same portion for each pixel but nonuniformly as shown in FIG. 4.
Since the portions where the disclination lines 9 appear are varied for each pixel, mottles appear on the displayed screen, for example white spots on a displayed black screen, due to the fact that the visible portions of the disclination lines 9 are not identical for each pixel.